1. Technical Field of the Invention
This invention relates to telecommunication systems and, more particularly, to a method of broadcasting a quality over-the-air multicast to a plurality of mobile terminals.
2. Description of Related Art
Multicasts are useful whenever there is a group of subscribers with a common interest, and information of interest to the group can be broadcast to the group in single transmission. For example, at a sporting event, multiple spectators with mobile terminals such as Personal Digital Assistants (PDAs) may desire to receive a broadcast of highlights or game statistics. The same data may be simultaneously transmitted in a broadcast transmission to all users, and no response is needed from the mobile terminals. The multicast process uses only one downlink channel, and no uplink channels, while leaving other channels in the network available for other users. Billing may be set up as a flat fee for a particular broadcast. The broadcast may be encrypted, or require the entry of a code to ensure that only subscribers who have signed up for the broadcast are able to receive it. In the Global System for Mobile Communications (GSM), a key can be set on the SIM card enabling the mobile terminal to receive the multicast.
Multicasting is efficient and uses minimal resources at the time of transmission, but the Quality of Service (QoS) (i.e., transmission quality) is dependent on the bearer. Multicast protocols are implemented at higher network layers that do not consider the transport media during the transmission, and the system operator does not know during the multicast whether the multicast is being received by the mobile terminals with sufficient transmission quality for the type of data being transmitted. Point-to-point unicasts, on the other hand, can ensure delivery by requiring an acknowledgment by the receiver. This approach works well in wired networks where the probability of packet losses is low. When a wireless network relays multicast packets over the air, however, the probability of packet loss is considerably higher. To compensate, data networks such as the General Packet Radio Service (GPRS) transfer the multicast packets to each wireless terminal as a point-to-point unicast packet. The Group Call Service (PTM-G), for example, which is defined in the GPRS standard within GSM, takes multicast inputs from reliable sources such as a corporate LAN (e.g., wired Ethernet) and sends them to wireless GPRS receivers as point-to-point unicasts. To achieve this, PTM-G uses a list of individual receivers that subscribe to a multicast transmission, and transmits a copy of the data individually to each receiver. An acknowledgment is sent from each receiver to the network. This type of transmission is not well suited for real-time, high-bandwidth transmissions since it utilizes re-transmissions at the radio link layer. Additionally, the method does not scale well when the number of receivers of the multicast service increases, and may require a substantial amount of network resources.
Alternatively, it is recognized that packets could simply be broadcast in area of cellular coverage without any acknowledgments from the receivers, hence without any guarantee of delivery. The problem with this approach is that the multicast service is a billable service, and a mechanism to measure and adjust transmissions according to the QoS delivered is essential.
Additionally, it should be noted that in GPRS, like most other networks, mobile terminals may have different capabilities in terms of the bandwidth that they support and their ability to support current applications. Current multicast procedures do not fully consider differing mobile terminal classes and their capabilities.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method of broadcasting a quality over-the-air multicast to a plurality of mobile terminals. The method should scale well when the number of receivers of the multicast service increases, should consider differing mobile terminal classes and their capabilities, and should allow the system operator to measure and adjust transmissions according to the QoS delivered, while not requiring that acknowledgments be received from each mobile terminal receiving the multicast. The present invention provides such a method.
In one aspect, the present invention is a method in a radio telecommunications network of broadcasting data in an over-the-air multicast to a group of mobile terminals. The method begins by determining a minimum transmission quality (TQmin) that a mobile terminal must receive for the type of data being broadcast, broadcasting a test transmission to the mobile terminals, and receiving a response from a mobile terminal representative of the group. The response includes an indication of the received transmission quality, and the method then determines whether the received transmission quality was greater than or equal to TQmin. If the received transmission quality was greater than or equal to TQmin, the data is broadcast in the over-the-air multicast to the group of mobile terminals.
In another aspect, the present invention is a method of broadcasting data in an over-the-air multicast to a group of mobile terminals which includes the steps of determining a minimum transmission quality (TQmin) that a mobile terminal must receive for the type of data being broadcast, and broadcasting a test transmission to the mobile terminals, the test transmission including a maximum transmission quality threshold (TQmax). The mobile terminals are instructed not to respond to the test transmission if their received transmission quality is above TQmax. This is followed by starting a response timer, and receiving a response from a mobile terminal representative of the group, the response including an indication of the received transmission quality. The method then determines whether the received transmission quality was greater than or equal to TQmin. If not, and the response timer has expired, TQmin is decreased and the test transmission is re-broadcast to the mobile terminals with the decreased TQmin. If the received transmission quality was greater than or equal to TQmin, however, the method broadcasts the data in the over-the-air multicast to the group of mobile terminals.
In yet another aspect, the present invention is a method of broadcasting data in an over-the-air multicast to a group of mobile terminals which includes the steps of determining a minimum transmission quality (TQmin) that a mobile terminal must receive for the type of data being broadcast, and broadcasting a test transmission to the mobile terminals, the test transmission including a maximum transmission quality threshold (TQmax). The mobile terminals are instructed not to respond to the test transmission if their received transmission quality is above TQmax. This is followed by receiving a response from a plurality of mobile terminals in the group, each response including an indication of the received transmission quality for the mobile terminal transmitting the response, and determining whether the received transmission quality was greater than or equal to TQmin for each of the received responses. If the received transmission quality was greater than or equal to TQmin for a predefined threshold number of the received responses, the data is broadcast in the over-the-air multicast to the group of mobile terminals.